1. Field of the Invention
This invention relates to a hole production tool and, more particularly, to a hollow rotary drill for cutting circular holes in material which has a relatively low compression strength, for example rubber, cork, wood, and laminated composite aramid fibre material such as that sold under the trademark KEVLAR by the DuPont Corporation.
2. Description of the Prior Art
Laminated aramid fibrous product, sold under the Trademark "KEVLAR" by DuPont Corporation is a "space-age" composite material which was developed for use as structural items in spacecraft, airplanes, boats, etc. The product has gained wide acceptance in these markets because of its high strength and low weight.
Machining KEVLAR material with a conventional drill bit frequently causes "fuzzing" about the edges of the hole being cut. A conventional drill bit will also cut the drilled material into small pieces, creating dust and small particles which can be physically irritating and possibly hazardous to human health. Accordingly, it is an object of the present invention to provide a new and novel hollow rotary drill for cutting a smooth hole in relatively low compression strength materials, particularly the KEVLAR material, without resultant "fuzzing" and without cutting the drilled material into small pieces.
Hollow rotary drills have been provided heretofore and have been utilized in the paper industry to produce holes in paper and other goods such as that disclosed in U.S. Pat. No. 1,557,464 granted to W. K. Mick on Oct. 13, 1925; and U.S. Pat. No. 1,219,677 granted to Walter Sparks on Mar. 20, 1917. As the prior art hollow rotary drill cuts through the stack of sheets, it will cut individual paper discs which will independently pass upwardly through the hollow drill shank. The cutting of a a hole in a stack of sheets of paper does not have the attendant material jamming problems associated with cutting a workpiece having a relatively low compression strength such as the aforementioned "KEVLAR" material. The individual discs cut from the individual sheets of paper will remain as individual discs and will not tend to bind together as will the drilled portion of the KEVLAR material.
Other prior patented hollow tubular cutting drills are illustrated in U.S. Pat. No. 2,564,451 granted to Ray A. Sandberg, et al on Oct. 14, 1951; U.S. Pat. No. 2,606,615 granted to Claude V. Pevey, et al on Aug. 12, 1952; U.S. Pat. No. 3,512,519 granted to Robert M. Hall on May 19, 1970; and U.S. Pat. No. 4,060,333 granted to John T. White on Nov. 29, 1977.
The hole production member constructed according to the present invention includes a hollow, rotary, tubular drill shank having, at its axially outer end, radially inner and outer cylindrical surface portions which axially outwardly converge to form an annular cutting edge. The severed material forms a core which passes axially inwardly, through a hollow passage provided in the drill shank, having a radially inwardly converging annular internal surface for radially compressing the core and upwardly passing it through a reduced diameter throat. The drill shank includes a hollow tubular passage, having a diameter greater than the diameter of the throat, above the throat to permit the radially compressed cored material to partially radially expand and yet freely axially pass without "binding up".
If the yield point of the drill shank material is too low, the annular cutting edge may radially outwardly billow, sometimes referred to as "bell-mouthing". Also, if the drill material is over-stressed or too hard, fractures may occur, and the material, starting at the cutting edge, will crack and fail. The construction of the hollow cutting drill according to the present invention is such as to inhibit the occurrence of "bell-mouthing" of the annular cutting edge due to high stress.
It has been found that sufficient material must be distributed at the cutting edge and throat area to reduce the operating stress in the drill shank to an acceptable level. This can be accomplished by decreasing the throat diameter, increasing the throat length, and increasing the cutting edge angle which, as used herein, will refer to the included angle between radially inner and outer, axially outwardly converging drill shank surfaces which converge to form the annular cutting edge.
Cutting thrust must be low enough to minimize tool stress and yet be within acceptable limits for drilling or cutting holes. The thrust is controlled, in part, by the cutting edge angle and the throat diameter. Once the angle is established, the throat and outside diameters of the cutting tool combine to produce a "projected area" of the tool as "seen" by the workpiece. It is this projected area which predominantly dictates the thrust requirements. With a fixed cutting edge angle, increasing the outside diameter of the cutting tool causes an exponential increase in thrust requirements, which can be reduced by a corresponding increase in throat diameter, which in turn increases the stress and the possibility of failure. These factor must be balanced relative to the cutting thrust requirements and the stress requirements.
Increasing the cutting edge angle is not necessarily efficient in stress reduction, since a large cutting edge angle increases the cutting thrust load, which in turn tends to increases the stress. Decreasing the throat diameter provides an increased cross-sectional area of material to reduce stress but has an attendant increased cutting thrust load.
It has been found that additional stress reduction without representing a significant impediment to the flow of workpiece material through the throat area, may be gained by lengthening the throat area without. Accordingly, it is an object of the present invention to provide a new and novel rotary hollow cutting drill of the type described, having a new and improved throat construction and a cutting edge angle which lies within a range of 30.degree. to 45.degree..
It is another object of the present invention to provide a new and novel rotary, hollow cutting drill of the type described, which includes a new and novel, material-compressing throat construction for radially compressing and axially elongating a cylindrical core of material removed from the workpiece.
A further object of the present invention is to provide new and novel rotary drill apparatus of the type described having a hollow drill shank, provided with a new and novel throat construction, which will permit the tool to be re-sharpened without changing the cutting edge angle, the relative dimensions of the diameter of the hole being cut, and the diameter of the throat.
As used herein, strain ratio is a term which means the distance which the material being cut diametrically compresses relative to the original diameter. For example, if the material being cut has an original diameter of 0.25 inches and it is diametrically compressed 0.05 inches, the result is a twenty percent strain. If the drilled core material, such as KEVLAR, is strained only a relatively small amount while passing through the reduced diameter throat area, the core will yield under compressive force but immediately, upon release of force, will spring back to its original shape and will frictionally bind on the drill shank side wall as the core flows upwardly through the drill. Accordingly the effects of plastic strain as well as elastic strain must be considered. With elastic strain only, upon release of the compressive forces, the material will spring back to its original shape. With plastic strain, however, the material, upon release of the compressive forces, will not return to its original dimensions. In some cases, both elastic and plastic strain occurs.
With apparatus constructed according to the present invention, the drill material forms a core which is compressed sufficiently to induce plastic strain, so that when the core material passes upwardly beyond the throat, it will not expand to its original size.
If the diametrical compressive strain is inadequate, the workpiece material core or slug removed will bind up in the exit area, creating increased cutting thrust and even "jamming" of the core in the tool. Strains below 25 percent will not effectively delaminate the KEVLAR, and strains over 35 percent create excessive drilling thrust and stress.
It has been determined that the severed KEVLAR material, when subjected to a certain radial strain, will delaminate the workpiece core sufficiently for it to flow, substantially uninhibited, upwardly through the internal passage provided in the cutting tool. Accordingly, it is another object of the present invention to provide a new and novel hollow rotary cutting drill of the type described, wherein the relative dimensions of the diameter of the cutting edge and the throat are such that the core material is subjected to a radially compressive plastic strain in the range of 25 percent to 36 percent.
It has also been found that the diameter of the tubular drill shank exit portion downstream or axially inward of the throat must be sufficiently larger than the throat diameter so as to allow for unhibited movement of the radially compressed core. It has been found that this factor is measured by the ratio of the throat strain to the exit strain required. It has been found that the tubular drill shank member must be constructed such that the predetermined diameter of the annular cutting edge, less the diameter of the throat diameter, relative to the predetermined diameter, less the diameter of exit diameter, lies within a range of 1.45 to 6.0.
Other objects and advantages of the present invention will become apparent to those of ordinary skill in the art as the description thereof proceeds.